Method for manufacturing an electret filter medium

ABSTRACT

The present invention relates to a continuous method for manufacturing an electret filter medium from dielectric material having an open or porous structure, said method comprising the steps of continuously feeding a web of dielectric material with a substantially closed dielectric foil adjacent to at least one major face thereof into a corona discharge device, reducing the thickness of the web of dielectric material and charging the web of reduced thickness dielectric material by means of a corona discharge and to apparatus for carrying out said method.

The present invention relates to a continuous method for manufacturingan electret filter medium from dielectric material having an open orporous structure, said method comprising the steps of continuouslyfeeding a web of dielectric material with a substantially closeddielectric foil adjacent to at least one major face thereof into acorona discharge device, reducing the thickness of the web of dielectricmaterial and charging the web of reduced thickness dielectric materialby means of a corona discharge and to apparatus for carrying out saidmethod.

A somewhat analagous but discontinous, high-temperature method formanufacturing an electret filter medium is disclosed in U.S. Pat. No.4,308,223. In said method, a filter web of polypropylene fibers of arelatively high basis weight, viz., 410 g/m² and a thickness of 2 mm ischarged by two effects, viz., orientation of dipoles and implantation ofcharges into the fibers. For optimum orientation of the dipoles, thecharging is conducted at a high temperature (120° C.) and consequently,requires quite a long time, viz., 15 minutes in total. The charging iscarried out by inserting the mat between two electrodes, one of which isprovided with a large number of corona points connected to a highDC-voltage. The other electrode is earthed and covered with a dielectricfoil for the purpose of preventing the ions produced by the corona fromflowing to earth. In order to inject negative CO₃ -ions, a negativecorona voltage is used. For the purpose of improving the uniformity ofthe charging of the filter medium, the corona points are positioned inclose proximity to one another on the upper electrode. The coronasoriginating from the points oppose one another and are operative over alimited distance (a few millimeters). The points are positioned a shortdistance above or in the filter material to be charged. High coronavoltages cannot be used because they cause sparking from the points tothe grounded electrode. The risk of sparking is notably greater when thepoints are pressed into the filter material. The sparks can cause ashort circuit which may produce holes in both the foil and the filtermaterial. The holes in the foil may even become so wide that the foil isrendered useless. The holes in the filter material allow the passage ofdust particles at an unacceptable rate.

Because the points as well as the upper electrode are at a high voltagethe arrangement can be unsafe. A further problem is that the filtermaterial is attached with screws onto the lower electrode so that partof the filter material remains uncharged and has to be discarded.

Furthermore, the charging is somewhat variable as appears from thepenetration data in Table II of U.S. Pat. No. 4,308,223. This variationfrom 1.3 to 7 mg. penetration is likely due to the non-uniform chargingof the filter material in which some parts of the filter material arenot charged optimally.

U.S. Pat. No. 4,375,718 discloses a continuous, room-temperature processfor manufacturing an electrostatically charged filter medium. In thismethod, a web of non-conductive, thermoplastic fibers is contacted oneach side with a more conductive web to form a combined composite web.The composite web is charged by corona charging elements of oppositepolarity on opposite sides of the composite web.

The present invention is directed to a continuous method ofmanufacturing an electret or electrostatic filter medium having superiorfiltration performance compared to media produced by prior art methods.The use of a substantially closed supporting dielectric foil has theadvantage of protecting against electrical breakdown or sparks duringthe charging process.

The method according to the present invention is very well suited for acontinuous process. Since the charge injection is so strong, it is notnecessary to aim at orientation of permanent dipoles and one may startwith dielectric materials without polar groups. These have a much higherinsulation resistance than do polar dielectric materials therebyconsiderably improving the long-term stability of the injected charges.Moreover, the method according to the invention can be carried out atroom temperature.

The charge injection is preferably done by means of a corona, generated,e.g., with thin tungsten wires. Although either a positive or a negativecorona can be used, the charging is conveniently carried out with twocoronas, a positive one at one side and a negative one at the other sideof the combination of dielectric material and dielectric foil. In thisarrangement one corona brings about the charge injection while the otherfunctions as a counter-electrode. Said arrangement has the advantagethat the risk of catastrophic breakdown through the dielectric foiland/or the material to be charged is low, because instead of a metalcounter-electrode, one of the coronas, viz., a plasma of air ions, actsas the counter-electrode. Over said plasma-type counter-electrode thedielectric foil can be moved rapidly and without friction, this being ofgreat importance in a continuous charging process.

Since a too heavy and/or a too thick filter web cannot be charged highlyenough to obtain the desired low penetration for fine dust particles,the electret filter web can be built up as a stack of a number of layerswhich are charged separately. Next, the layer structure can beeliminated by, e.g., needle tacking or heat sealing.

The superior charging of the dielectric material is achieved if thethickness of the dielectric material is reduced during charging. It ispossible to permanently reduce the thickness of the dielectric materialby compressing it before charging so that it does not recover. Thismight be the case in the fabrication of a molded shape from an opendielectric material. The thickness of the material can also be reducedby stretching in the direction of its length and/or width. This appliesin particular to a material that can be elastically deformed, such asfoam. Temporary stretching during charging has the additional advantagethat the area weight of the material is decreased.

Reduction in thickness can be achieved by pressing an open gauze or netonto the dielectric material, e.g., by means of rollers over which thegauze is run being positioned so as to press against the web. Also, itis possible to apply a pneumatic overpressure to a substantially closedfoil lying on top of the dielectric web and by this means achievecompression.

Reduction in thickness is preferably achieved by placing the dielectricmaterial into a substantially gas-tight space, at least one of theboundaries of which is flexible and is perpendicular to the thickness ofthe dielectric material, and drawing a partial vacuum in the sealedspace. Said gas-tight space can comprise an envelope of substantiallyclosed dielectric foil immediately adjacent to the open dielectricmaterial.

Surprisingly, and unexpectedly, it has been found that the result ofsaid charging under partial vacuum, as evidenced by filter penetrationdata, is better than achieved by charging under other methods ofcompression. Moreover, the vacuum method allows a high reduction inthickness (a factor of 5 or more) to be achieved conveniently.

When a dielectric filter material with a curved surface, such as isused, e.g., in respirators, is to be charged, the dielectric material isfirst preshaped, whereupon it can be charged according to the invention,e.g., in a partial vacuum. The shaping is normally carried out underpressure and at a high temperature. A precharged filter web may therebylose part of its charge; such loss can be avoided by carrying out thecharging after the shaping operation.

It is clear that the charging of the dielectric material is accompaniedby simultaneous charging of the adjacent substantially closed separatingfoil. When the same separating foil is to be used in successive chargingof a number of pieces of dielectric material, it appears thatdischarging said foil after each charging operation improves the chargeon successive pieces.

Surprisingly, further improvement in charging is achieved if theseparating foil, before being used for charging the dielectric material,is charged with a polarity opposite to that used for charging thematerial.

It has further been found that the dielectric material is charged morebipolarly, if it is pre-exposed to a charge with a polarity opposite tothat used during charging.

A similar improvement in the bipolarity of the dielectric material canbe achieved if before it receives its final charge it is subjected to anAC corona. Furthermore, it appears that filter material charged in anessentially gas-tight space at a lower pressure or at an overpressure iscompletely bipolar.

Polarity of the dielectric material may be determined by reading thesurface potential of the web using a non-contacting probe such as theMonroe Isoprobe Electrostatic Voltmeter. Bipolar webs will showessentially zero readings due to the nearly complete compensation ofcharges of each polarity in the material by equal concentrations ofcharges of the opposite sign.

Because of the improved bipolarity of the dielectric filter material,the polarity of the charged particles to be filtered is not ofimportance and the strongly inhomogeneous fields produced in the filterresults in improved capture of uncharged particles.

For high filter efficiency the dielectric filter material shouldpreferably consist of very fine fibers.

The invention further relates to equipment for the continuousmanufacture of an electret filter web, which equipment, in oneembodiment, comprises a corona device in the corona of which anessentially closed dielectric foil extends substantially at right anglesto the corona field. Said apparatus is characterized by the dielectricfoil forming part of an endless belt guided around rotating rollers andby the corona device having at least one positive and one negativecorona spanning between said part of the belt, in which span thedielectric material that lies against the belt is moved forward betweenthe coronas.

In another embodiment of the equipment, a grounded metal electrode islocated opposite the corona. The electrode comprises a rotating metalroller or a belt. The dielectric foil is placed on the circumference ofthe roller or on that side of the belt which faces the corona. Thedielectric material situated on the dielectric foil is therebytransported through the corona. The dielectric foil may be adhered as acover on the surface of the roller or belt.

It will be apparent that the dielectric material can also be transportedbetween the dielectric foil and the metal roller or belt, the foil beingpressed in the direction of the roller or belt so as to compress thedielectric material.

The invention also relates to an apparatus for manufacturing, in seriesproduction, a large number of electret filter webs from dielectricmaterial with an open structure comprising a corona device in the coronaof which a substantially closed dielectric foil mainly extendsperpendicularly to the corona field. Said apparatus is characterized inthat it incorporates a device for supplying foil and the dielectricmaterial to be charged, this device is followed by a vacuum packingdevice, which packs the foil and the dielectric material into evacuatedpackages, and in that the corona device has a positive and a negativecorona, between which the evacuated packages are transported.

Another embodiment of the present invention is characterized in that theelectrode is a rotatable drum, the cylindrical circumferential wall ofwhich is provided with holes and in that the blocking foil is guided ata certain distance from the cylindrical outer surface of the drum toobtain a feed-through space for the dielectric material, and in whichthe foil is drawn to the drum by evacuating the feed-through spacethrough the holes in the drum wall opening into said space.

Preferably, the foil is an endless belt guided over rotatable rollers.

Still another embodiment according to the invention is characterized inthat the interior of the drum opposite the holes in the drum wallopening into the feed-through space of the dielectric material containsa stationary body which, in the direction of the axis of the drum, hastwo sealing faces in close contact with the inner side of thecylindrical drum near the outermost holes opening into the feed-throughspace and in that the surface of the body extending between the sealingfaces and facing the holes opening into the feed-through space is spacedfrom the drum wall so as to create a closed suction space connected to asuction conduit.

The invention will now be elucidated by drawings.

FIG. 1. shows an embodiment of an apparatus for charging the dielectricmaterial according to the invention;

FIG. 2 illustrates an apparatus allowing the method according to thepresent invention to be carried out continously;

FIG. 3 is an elaboration of the embodiment shown in FIG. 2;

FIG. 4 shows another embodiment for carrying out the method according tothe invention continously;

FIG. 5 presents an apparatus for carrying out the method according tothe invention continously;

FIG. 6 shows another embodiment for carrying out the method according tothe invention continuously;

FIG. 7 is a graph showing filter penetrations for electret filter matsof the invention versus the number of layers charged separately; and

FIG. 8 is a graph illustrating the penetrations obtained in percentversus overpressure and underpressure, respectively, the charging havingbeen carried out with four separate layers.

The invention will be described hereafter with reference to electretfilters made from dielectric fiber material. It is clear, however, thatother dielectric materials having an open structure, for example,dielectric foam, porous membranes, sintered powder, etc., can likewisebe charged according to the present invention with the same advantages.It is also clear that the charged material may be used for purposesother than filtration.

Further, the fibers may be fine and/or coarse and take any shape such asround, lobed, rectangular, hollow and so on. Furthermore, it is alsopossible to charge staple fibers, non-woven, spun, melt-blown,solvent-blown or sprayed fibers or a mixture of several of these fibers.The dielectric filter material may comprise different layers, e.g., onecoarse layer of fibers and a layer of fine fibers. If desired, severaldielectric materials can be used for the layers.

The current trend in the design of filters is focussed on capturing notonly coarse but also fine dust more and more effectively. This is ofimportance both in air conditioning systems and for personal protectionbecause particles of a dimension less than one micrometer are the mostdangerous. These particles can be inhaled and often contain heavymetals. Further, fine dust should be excluded from dust-free rooms("clean rooms") in which micro-electronic components are manufacturedand from intensive-care units in hospitals, etc. Further, in manymanufacturing processes very fine dust is produced and atmospheric dustalso contains many submicron particles which may be injurious to health.

In conventional fibrous filters, fine dust is captured effectively onlyif the fibers are very fine. The ability to capture fine particles isvastly improved by applying an electrostatic charge on the fibers. Thisforms the basis of the electret filters according to, e.g., the DutchPat. No. 160,303 or U.S. Pat. No. RE. 30, 782.

In the past, permanent electrostatic charging of a dielectric materialwith an open or porous structure, particularly fibers, has posedproblems arising from undesirable dielectric breakdown. According toDutch Pat. No. 160,303 or U.S. Pat. No. Re. 30,782, said breakdown isavoided by using a closed dielectric foil which is first stretchedlengthwise, and then charged, and then fibrillated into fibers.

For a number of applications, fibers carrying an electrostatic chargeneed not be fine. The fibers in the electret filters described in DutchPat. No. 160,303 or U.S. Pat. No. Re 30,782, which patent isincorporated herein by reference, are relatively coarse because they aremade from split fibers (10×40 micrometers). Although these fibers carrya high charge, they are deleteriously affected by very fine dust. As aresult they are not especially useful for some long term applicationssuch as in air conditioning systems.

The present invention opens new perspectives by making it possible tocharge existing fiber webs made of micro-fine fibers. Several goodmethods are known for making micro-fine fibers and mixtures ofmicro-fine fibers and staple fibers. See e.g., German"Offenlegungsschriften" 2,328,015, 2,032,072 and 2,620,399; U.S. Pat.No. 4,230,650 and German "Offenlegungsschrift" 2,940,170. See also U.S.Pat. Nos. 3,016,559 and 4,118,531 and Van A. Wente, "SuperfineThermoplastic Fibers", Industrial and Engineering Chemistry, Vol. 48, p.1342 et seq. (1956).

Referring now to the drawings, FIG. 1 shows an embodiment of anapparatus with which the method according to the invention can becarried out.

In this apparatus, two corona plasmas 1 and 2 are used, a positive and anegative one. Between the two corona plasmas 1 and 2, a substantiallyclosed dielectric foil 3 is placed carrying the filter mat 4 to becharged. The dielectric foil 3 functions as a barrier separating thepositive and negative ions, thereby blocking their mutualneutralization. Said foil is therefore referred to as a separating orblocking foil. The use of a corona, i.e., a plasma of air ions, as acounter-electrode instead of a metal electrode reduces the risk of sparkbreakdown through the separating foil. In addition, the combination offilter web and foil can be transported between the coronas at highspeeds. The reduction of the risk of spark breakdown and thefrictionless transport allow the use of a very thin (e.g., 2 micrometersthick) dielectric foil. This increases the charging of the fiber matbecause the voltage loss across a thin foil is low. The use of thinfoils is particularly useful in charging thin fiber fleeces.

The corona plasmas 1 and 2 are produced by thin tungsten wires 5 and 6positioned respectively above and below the combination of the filtermat 4 and foil 3. The wires 5 and 6 are connected to positive andnegative voltages of, e.g., 7 kV supplied by the voltage sources 7 and8, respectively. By positioning the wires perpendicularly to the lengthdirection of the filter mat, a uniform charge is deposited. Incomparison to the known charging method with corona points, the coronawires have the advantage of being less vulnerable to damage, less easilycontaminated and less subject to spark erosion. Moreover, wires afford abetter charging geometry. The corona device is equipped with twogrounded plates 9 and 10 which strengthen the ionization of the air sothat many more ions are available for injection. Further, the groundedplates 9 and 10 render the arrangement electrically safe.

In the apparatus of FIG. 1, a wide-mesh dielectric gauze (not shown) maybe pressed mechanically onto the dielectric material to be charged andseparating foil 3. For that purpose, the foil 3 is stretched across aframe (not shown) which is mounted in a fixed position. Similarly, thegauze is stretched across a frame (not shown) and pressed in thedirection of the foil.

Preferably, the charging of the filter material is carried outcontinuously, the filter material, as far as it is inside the coronadevice, being compressed continuously by the gauze turned round by meansof a roller system (not shown).

The corona charging used is fast (may be completed in one second orless) and can be carried out at a range of temperatures, but preferablyat room temperature.

The charges implanted into the dielectric material by the coronas areenergetically bound to structural defects in the material, in otherwords they are captured in so-called "traps" (capturing centers). Asnoted, the charge storage in the traps can normally be achieved at roomtemperature. Only if the material also contains shallow traps inaddition to deep traps, is it preferable to carry out the charging at ahigher temperature, because high temperatures favour the filling of thedeep traps, the shallow ones remaining empty.

The charge injection is so strong that one needs no longer to strive forthe orientation of permanent dipoles in the filter material, so that onemay start from dielectric materials without polar groups. Owing to thehigher insulation resistance of said non-polar materials compared withthat of polar materials, the stability of the injected charges is muchbetter in the long run.

Several dielectric materials have been found to be suitable includingsuch polymers as: polypropylene, linear low density polyethylene,polymethylpentene, polytetrafluoroethylene, polytrifluorochloroethylene,polystyrene, polycarbonate, polyester and others.

Superior charging can be achieved by compressing the dielectric materialin the direction of its thickness during charging. This compression ispossible because a filter contains much air, the filling or packingdensity, (volume of fibers/total volume) frequently being only a fewpercent. The filters can often be compressed to a thickness of one fifthor less of the original thickness.

The compression can be achieved in different ways. First, the filter matcan be compressed mechanically on the blocking foil, for example bymeans of a wide-mesh gauze. Further, the filter mat can be chargedbetween two foils instead of one. In this case, the filter mat can becompressed by applying a pneumatic overpressure to the foils.Optionally, the filter mat can be compressed permanently, e.g., bycompressing it in a high-pressure press, if desired at a hightemperature, or by feeding it between hot-press rollers (calander).Compression can also be achieved in the process of molding the filtermaterial into a shape such as a respirator.

Exceptionally good results are obtained when compression of the filtermat or fiber fleece is achieved by a pressure reduction. In that case,the fiber fleece is enclosed in a substantially gas-tight space, ofwhich at least one of the boundaries (a major face) perpendicular to thedirection of the thickness of the dielectric material is flexible. Byreducing the pressure in said space the fiber fleece is compressed. Theboundaries of said space may consist of, e.g., an upper and a lowerfoil. One of these foils may have, at least locally, a low porositywhich allows one to create a certain underpressure within the spacebetween the two foils. It appears that such low porosity has no effecton the blocking activity of the separating foil. Alternatively, the airmay be sucked away at the edges of the filter mat, in which case theseparating foil need not be porous.

The boundaries of the space may be formed by a so-called blown foil(tube) enclosing the fleece entirely and also capable of functioning asa separating foil during the charging process. The air is sucked away atthe open end of the blown foil. If necessary, said blown foil may serveas packing later on, for protection against moisture and dust.

Surprisingly, it has been found that for the same reduction in thicknessthe charging at a reduced pressure gives a better result, i.e., a lowerparticle penetration, than do the other forms of compression.

The great advantage of the invention, in particular when compression isused, can be seen in Tables A, B, C, and D. Testing is according toBritish Standard 400, a test of the penetration of a standard dispersionof sodium chloride particles generated by nebulizing an aqueous solutionof NaCl. The aerosol concentration is measured in a hydrogen flame. Airflow is at a velocity of 20 cm/sec.

The quantity Q is a figure of merit for filter media definedmathematically by the expression ##EQU1## where %P is the percentpenetration, ΔP is the pressure drop in Pascals and ln indicates thenatural logarithm. This figure of merit is always positive and increaseswith reduced penetration. Conversely, as pressure drop increases, thevalue of Q is reduced.

In the event that a filter web is made thicker by the addition ofmaterial, then the penetration of relatively fine particles is found tobe well approximated by the mathematical expression

    %P=100e.sup.-k·W                                  (2)

where W is the basis weight, k is a constant and e is the base for thenatural logarithm. Basis weight is defined as the weight per unit areaof web, e.g., in grams per square meter. (This assumes that theadditional material is of the same fiber, fiber orientation and degreeof compaction as the original material and that interfacial effects anddepth loading effects can be neglected).

Similarly, the pressure drop may be computed from the expression

    P=k'·W                                            (3)

where k' is a constant and W is the basis weight.

Combining equations (1), (2) and (3) results in

    Q=k/k'                                                     (4)

This demonstrates that Q is an index which is independent of the directeffect of the basis weight on filter performance. Thus, Q may be used tocompare the filtration performance of webs of different basis weights.(See William C. Hinds, Aerosol Technology: Properties, Behaviour, andMeasurement of Airborne Particles, John Wiley and Sons, New York,Chapter 9. If Q is different between two webs, it can be because eitherk or k', or both are different. k might be different because the fibersare differently charged. This is seen in the first two lines of Table Ain which the beneficial effect of charging at reduced area weight andthickness is seen. Similarly, k' can be increased if the web ispermanently compressed. This can be seen in the increased pressure dropsobserved in lines 5 and 6 of Table A.

                  TABLE A                                                         ______________________________________                                                         penetration test with NaCl at                                                 20 cm/sec                                                           charging  (for separate charging, testing                                     total         was done after reassembling)                                      basis   thick-  basis pressure                                                                             pene-                                            weight  ness    weight                                                                              drop   tration                                                                             Q                                 filter   g/m.sup.2                                                                             mm      g/m.sup.2                                                                           Pa     %     Pa.sup.-1                         ______________________________________                                        split fibers                                                                  (not carded)                                                                  charged as                                                                             203     6.5     203   23     62    .021                              one mat                                                                       in 4 layers                                                                             36     1.5     142   18     30    .067                              charged                                                                       separately                                                                    charged in                                                                             196     1       196   23     31    .051                              vacuum as                                                                     one mat                                                                       in 4 layers                                                                             41     0.2     165   20      5    .150                              charged                                                                       separately                                                                    in vacuum                                                                     split fibers,                                                                 compressed                                                                    permanently                                                                   charged in                                                                             188     0.8     188   38     33    .029                              vacuum as                                                                     one mat                                                                       in 4 layers                                                                             40      0.35   161   41      5    .073                              charged                                                                       separately                                                                    in vacuum                                                                     ______________________________________                                    

Table A relates to a non-carded filter web from polypropylene splitfibers. These are relatively coarse and rectangular in cross section(dimensions 9×45 micrometers). The filter material was charged in twoways: i.e., without compression with one separating foil; and in vacuum,with compression between two separating foils. The charging was carriedout on an apparatus similar to that illustrated in FIG. 1 in about 1second at 25° C. with corona voltages of ±7 kV and with one or two 2micrometer thick blocking foils of Mylar, the two foils forming asubstantially gas-tight enclosure for charging in partial vacuum (onlyone foil being shown in FIG. 1).

The table shows that free charging in four layers instead of one reducesthe salt penetration from 62 to 30%. A comparable result was obtainedwhen the entire mat was charged at one time in a partial vacuum of 30kPa. A much better result was obtained when the filter material wascharged in a partial vacuum of 30 kPa in four layers, then the saltpenetration was reduced to a mere 5%.

In the two final examples the filter material was compressed permanentlyin a press under a high pressure of 11.8 MPa at 25° C. which preventsthe material from regaining its original height after charging. Theseexamples also reveal the surprising benefit of charging in four layersin a vacuum.

                                      TABLE B                                     __________________________________________________________________________                       penetration test                                                              NaCl at 20 cm/sec                                                 total                                                                             total                                                                             total    penetration                                                                             Q                                                  number                                                                            basis                                                                             thick-                                                                            pressure                                                                           before                                                                             after                                                                              before                                                                             after                                  filter of  weight                                                                            ness                                                                              drop charging                                                                           charging                                                                           charging                                                                           charging                               material                                                                             layers                                                                            g/m.sup.2                                                                         mm  Pa   %    %    Pa.sup.-1                                                                          Pa.sup.-1                              __________________________________________________________________________    very fine                                                                            1    34  0.45                                                                             43.4 70   6.7  .0082                                                                              .062                                   fibers 2    61 0.9  72       1.8       .056                                          3    99  1.35                                                                             105       1.5       .040                                          4   133 1.8 136       0.7       .036                                   fine and                                                                             1   146 2.9  76  50   2.3  .0066                                                                              .050                                   coarse 2   318 5.8 149  25   0.5  .0093                                                                              .036                                   fibers 3   450 8.7 258  16   0.3  .0071                                                                              .023                                   mixed                                                                         respirator filling                                                            (fine fibers +                                                                       4   428 3   447   9   1.2  .0054                                                                              .010                                   carrying                                                                      web)                                                                          __________________________________________________________________________

In Table B, the filter material was made of polypropylene fibers andcharged in one layer or a stack of layers superimposed. The charging wascarried out on an apparatus similar to that illustrated in FIG. 1 in apartial vacuum of 30 kPa at room temperature, in about 1 second, andwith corona voltages of ±7 kV, and two separating foils of 2 micrometerthick Mylar, the latter foils forming a substantoially gas-tight space(only one foil being shown in FIG. 1).

The very fine fibers are melt blown polypropylene microfibers producedby the method described in Van A. Wente, "Superfine ThermoplasticFibers", Industrial and Engineering Chemistry, Vol. 48, pp. 1342 et seq.(1956). The fibers are a mixture of sizes ranging from submicrometer toseveral micrometers in diameter.

The fine fibers are a mixture of melt blown polypropylene fibers asdescribed above and polypropylene staple fibers, of approximately 25micrometers diameter, added in a proportion of approximately 30% byweight, using a method described in U.S. Pat. No. 4,118,531.

The respirator filling is a very fine fiber similar to the above veryfine fiber taken from a 3M Company Type 8710 dust and mist disposablerespirator.

Table B demonstrates that charging and compression in a partial vacuumaffords efficient charging of filter materials of a high basis weightand high thickness. This is borne out particularly by the results forthe filter material composed of a mixture of fine fibers and coarsefibers. For example for a basis weight of 318 g/m² and a filterthickness of 5.8 mm, the salt penetration decreases from 25% to 0.5%after charging in a partial vacuum. Even material having a basis weightof 450 g/m² and a thickness of 8.7 mm can still be charged.

The test results listed in Table B for very fine fibers show that fourlayers having a total basis weight of 133 g/m² and a total thickness of1.8 mm can be charged simultaneously, whereby the salt penetrationdecreases down to 0.7% after charging. The laminated filter material offace masks can also be charged quite efficiently in a stack of fourlayers. The salt penetration then falls from 9 to 1.2%.

Although, the use of compression during charging (or before charging butpermanently) greatly improves the filter efficiency (defined in percentas 100 minus the penetration in percent), it still remains profitable tobuild up a filter web from layers, charged separately under compressionand later assembled into one web. This is demonstrated by Tables C andD.

                  TABLE C                                                         ______________________________________                                                    penetration test with NaCl at 20 cm/sec                                                thick-  penetration (with                                              pres-  ness    division, measured                                             sure   each    after stacking all                               during charging                                                                             drop   layer   layers together)                                                                         Q                                     compression                                                                            division Pa     mm    %          Pa.sup.-1                           ______________________________________                                        no       no       20     12    60         .026                                no       in 4     18     6     30         .067                                         layers                                                               compressed                                                                             in 4     18     2.6   20         .089                                twice    layers                                                               vacuum   in 4     20     .2     6         .141                                (1.33 kPa)                                                                             layers                                                               overpressure                                                                           in 4     22     .4    14         .089                                (32 kPa) layers                                                               compressed                                                                             in 4     71     .4     7         .037                                permanently                                                                            layers                                                               ______________________________________                                    

Table C relates to the filter properties of polypropylene split fiberfilters of 190 g/m², compressed during charging and/or charged in fourlayers. The charging was performed at 25° C. in an apparatus similar tothat illustrated in FIG. 1 in about 1 second with corona voltages ofabout ±7 kV. The compression was achieved respectively by compressionwith an open gauze lying over the filter, by vacuum suction and underthe action of an overpressure. In the fourth and fifth lines of theTable, the filter web was enclosed between two blocking foils sealedtogether. In the fourth line, the space betwen the two foils wasevacuated down to a pressure indicated in the Table, whereas in thefifth line the two foils were pressed together by an overpressure of 30cm Hg produced in a chamber enclosing the entire apparatus withprovision made for escape of air from the otherwise gas-tight spacelimited by the blocking foils. Lastly, a filter was compressedpermanently by compression in a high-pressure press at room temperatureunder a pressure of 11.8 MPa. In the latter case, the filter web did notrecover, so the pressure drop was higher, viz., 71 Pa instead of 20 Pa.The results of free charging without compression show that the saltpenetration decreases from 60 to 30% by dividing the filter web intofour layers. A two fold compression reduces this to 20%. Vacuum andcompression by overpressure are more effective, for they result in adecrease in penetration to 6% and 14% respectively.

                  TABLE D                                                         ______________________________________                                                      penetration test with NaCl at                                                 20 cm/sec                                                       charging        total                                                                 basis   thick-  basis pressure                                                                             pene-                                    filter  weight  ness    weight                                                                              drop   tration                                                                             Q                                  material                                                                              g/m.sup.2                                                                             mm      g/m.sup.2                                                                           Pa     %     Pa.sup.-1                          ______________________________________                                        split fibers                                                                  discharged              160   19     90     .0055                             with X-rays                                                                   charged with                                                                          36      1.5     142   18     30    .067                               B-foil                                                                        charged in                                                                            41      0.2     165   20      5    .150                               vacuum                                                                        fine fibers                                                                   discharged              165   75     52     .0090                             with X-rays                                                                   charged with                                                                          144     2.8     144   60     18    .029                               B-foil                                                                        charged in                                                                            146     0.9     146   76      2    .052                               vacuum                                                                        very fine                                                                     fibers                                                                        discharged               34   47     85     .0035                             with X-rays                                                                   charged with                                                                          33      0.4      33   41     15    .046                               B-foil                                                                        charged 33      0.2      33   49      5    .061                               in vacuum                                                                     ______________________________________                                    

Table D lists the charging results for three types of filter material ofpolypropylene, viz., split fibers, fine and very fine fibers. The fineand very fine fibers are the same as described for Table B. The charginglasted about 1 second and was done at 25° C. with corona voltages of ±7kV, and with a blocking foil for 2 micrometer thick Mylar, again in theapparatus as illustrated in FIG. 1, except that sealed bags of blockingfoil were used for the vacuum charging. Before being charged, the splitfiber web was divided into four layers, which were reassembled into asingle fiber web after being charged separately. The Table again showsthat the NaCl penetration of the split fiber webs are lowest when vacuumcharging is used. The latter method reduces the penetration from 90 to5%. This may be due to the high compression of the filter web, whosethickness decreases from 1.5 to 0.2 mm. The X-ray discharge is used toprovide samples wnich are known to be completely uncharged.

The filter webs of fine and very fine fibers were not divided. Thepenetrations achieved for these webs (of 146 g/m² and 33 g/m²) were 2%and 5% respectively after vacuum charging. Their free charging, i.e.,without compression, also gave quite good results, viz. saltpenetrations of 18% and 15% respectively.

By the method of the invention it is even possible to charge existingfilters permancntly to produce electret filters. An example of this isgiven in Table E.

                                      TABLE E                                     __________________________________________________________________________                              penetration test                                                              NaCl at 20 cm/sec                                                             total                                                           basis         pressure                                                                           total                                                      weight   post-                                                                              drop penetration                                                                         Q                                        filter material                                                                           g/m.sup.2                                                                              charging                                                                           Pa   %     Pa.sup.-1                                __________________________________________________________________________    highly efficient bag                                                          filter                                                                        covering web                                                                              82       no   164  22    .0092                                    micro-fine polycarbonate                                                      fibers      72    203                                                         carrier web 49                                                                micro-fine polycarbonate                                                                  121      vacuum                                                                             164  4.5   .019                                     fibers + carrying fleece                                                      __________________________________________________________________________

Table E lists the test results for a highly efficient commerciallyavailable bag filter. The micro-fine fiber material is manufactured byCarl Freudenberg according to the method of British Pat. No. 1,346,231.Said filter was subjected, according to the invention, to post-chargingusing apparatus such as illustrated in FIG. 1 together with sealed,evacuated covering foils. The bag filter consisted of a carrying webcarrying the filter layer proper consisting of micro-fine polycarbonatefibers, which in their turn are protected with a covering web. The Tableshows that post-charging reduces the penetration from 22% to 4.5%.

Since the charging is carried out with two corona plasmas, not withrigid charging electrodes, even pre-shaped non-planar filter materialcan also be charged. Such materials are used, e.g., in respirators. Thepre-shaping into a "cup" is often carried out under pressure and at ahigh temperature. Charged filter material may lose thereby some of itscharge. This is avoided by carrying out the charging after the shaping.

The charging is preferably conducted in a partial vacuum, the pre-shapedfilter material being enclosed in a thin blown foil tube in which thepressure is reduced. The coronas are located above and below the filtermaterial. For optimum charging, their shape should be adapted to that ofthe curved mask. If necessary, the corona on the concave side of a facemask can be substituted with a thin metal foil. Alternatively, theseparating foil can be metallized on the concave side. These foils havethe advantage of adapting themselves nicely to the curved surface of themask, particularly when a vacuum is applied.

Another option is to position a corona on the hollow side, a current ofair being circulated in that cavity in such a way that the ions areblown against the separating foil.

A respirator generally consists of three layers: a protecting coveringweb, the filter web proper made of micro-fine fibers and a carrying web.The filter efficiency of the covering web and the carrying fleece islow, because these consist of relatively coarse fibers. The coveringlayer is primarily intended for imparting a certain rigidity to themask. It is preferable not to charge the relatively thick and heavycovering layer, because such a charging otherwise would impair that ofthe filter layer proper (which is thin and light). Table F includes anexample of a complete respirator post-charged in a vacuum. In this case,a 3M Company Type 8710 dust and mist disposable respirator was used.Again the apparatus and charging conditions are similar to those used inprevious examples.

                                      TABLE F                                     __________________________________________________________________________                             penetration test                                                              NaCl at 20 cm/sec                                                             total                                                            basis        pressure                                                                           total                                                       weight  post-                                                                              drop penetration                                                                         Q                                         filter material                                                                           g/m.sup.2                                                                             charging                                                                           Pa   %     Pa.sup.-1                                 __________________________________________________________________________    face mask                                                                     covering fleece                                                                           202                                                               microfine polypropylene                                                                    60                                                               fibers           302                                                                              no   127  23    .012                                      carrier fleece                                                                             40                                                               covering fleece +   vacuum                                                    fibers + carrier                                                                          302     charging                                                                           127  17    .014                                      fleece                                                                        fibers              vacuum                                                    +           100     charging                                                                           127   2    .031                                      fleece                                                                        __________________________________________________________________________

Evidently, post-charging reduces the salt penetration by a factor of1.4. A much higher gain is obtained when instead of the completeready-made filter, only the polypropylene fiber fleece is chargedtogether with the carrying web. This gain is made owing to the heavycovering fleece being composed of coarse fibers and so hardlycontributing to the capture of fine dust, whereas it requires a largeportion of the charging voltage.

Table G, which illustrates the results of the simultaneous vacuumcharging of a stack of three superposed layers of fine fiber material asbefore, shows that even when vacuum charging is used, it still pays tobuild up a filter mat from thin layers charged separately.

Again in these example apparatus as shown in FIG. 1 was used, withsealed, evacuated foils and corona operating as in the previousexamples. Charging is at room temperature.

                  TABLE G                                                         ______________________________________                                                                        penetration                                                           basis   NaCl at 20                                    filter                  weight  cm/sec                                        material  filter layer  g/m.sup.2                                                                             %                                             ______________________________________                                        fine fibers                                                                             top           140      9.5                                          charged in                                                                              center        130     23                                            vacuum    lower         140     13.5                                          ______________________________________                                    

Table G shows that the penetration of the central layer is lesssatisfactory (higher penetration) than that of the outer ones. The lowercharging of the central layer will have less effect on the totalperformance when the stack has a lower thickness, in other words, when athin web is charged. The thin webs are stacked together to form a singlefilter web after being charged separately, whereupon the coherence ofthe layers is improved by, for example, needle tacking or heat sealing.

In view of what has been said, the fiber web should be as thin and/or aslight as possible and/or the compression should be as high as possiblein order to acquire the highest possible charging. However, littleadditional benefit is gained by exceeding a certain number of layers, orusing less than a certain underpressure or more than a certainoverpressure. This is shown by the curves in FIGS. 7 and 8.

The curves in FIG. 7 represent test results for a filter web ofpolypropylene split fibers with a basis weight of 185 g/m² under severalcharging conditions. The ordinate gives the penetration data in percent,measured in a NaCl test at 20 cm/sec. The abscissa gives the number oflayers, with which the filter is charged. Curves a, b, c and d relateto, respectively, free charging, charging with a twofold compression,charging at a partial vacuum of 30 kPa and charging of a filterpermanently compressed previously at a pressure of 11.8 MPa at roomtemperature.

FIG. 8 presents the test results for a similiar filter mat as in FIG. 7.The same NaCl test was used. Along the ordinate again the penetration inpercent is plotted, whereas the abscissa gives the overpressure orunderpressure in kPa. Curve a pertains to charging in four layers underoverpressure, whereas curve b gives the results for charging in fourlayers in a partial vacuum; in both cases the filter medium was chargedbetween two blocking foils of 2 micrometer thick Mylar, at 25° C. withcorona voltages of ±7 kV, in an apparatus similar to that shown in FIG.1.

The separating foil should preferably make intimate contact with thematerial to be charged, particularly when this is a thin web. Said closecontact is promoted by taking a thin easyily pliable separating foil,because such a foil conforms well to the contours of the web, the moreso because it is aided in doing so by the electrostatic attractionbetween the fiber web and the foil.

If, for example, the material to be charged is a web of flat splitfibers, the web is preferably tensioned in the plane of the separatingfoil so as to improve mutual contact and thereby optimize the chargingof the fibers. The use of thin separating foils for charging thin fiberwebs is advantageous, because they give a lower voltage loss than dothick foils.

The separating foil may consist of a thin foil of a variety ofinsulating polymers, e.g., polyethyleneterephthalate (PET),polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE).PET of 2 micrometer thickness and polypropylene of 10-50 micrometerthickness afford good results.

Furthermore, it appeared that the thinner the webs to be charged, thethinner the separating foil should be.

Decay experiments with filter material of polypropylene split fiberscharged in a vacuum have shown that the penetration rises from 3.2 to 5%when the filter material was exposed to a humid atmosphere with arelative humidity of 100% at an elevated temperature (45° C.) for 34days. The stability therefore seems to be very good.

When in successive charging operations the same blocking foil is used,said foil is found to be charged strongly with a polarity correspondingto the polarity of the charge carriers to be implanted. Discharging thefoil between successive charging operations, preferably by means of anAC corona, affords an improvement in the charge by about 10%. Seemingly,the charge of the foil, where it contacts the fibers, counteracts thecharging of the fibers. Surprisingly, it is found that opposite polingof the foil raises the charge on the fibers further. By opposite polingis meant that, before charging the web, the foil acquires a polarityopposite to that of the final charge carriers to be implanted.

Table H shows the effect of said respective discharging and oppositecharging operations on the blocking or B-foil.

                  TABLE H                                                         ______________________________________                                                                          penetration                                                    basis   pressure                                                                             NaCl at 20                                  filter             weight  drop   cm/sec  Q                                   material                                                                             B-foil      g/m.sup.2                                                                             Pa     %       Pa.sup.-1                           ______________________________________                                        charged                                                                              not discharged                                                                            195     26     5.8     .110                                split  discharged  200     25     4.8     .121                                fiber  charged (with                                                                             170     15     5.4     .195                                fleece opposite                                                                      polarity)                                                              ______________________________________                                    

The results shown in Table H were obtained at a charging temperature of25° C., a feeding speed of 10 m/min., corona voltages of ±7 kV, and acorona length of 20 cm in an apparatus of the type illustrated in FIG.2, or for the case of discharged or oppositely charged foil, in FIG. 3.

From a quick glance at Table H, it appears that charging of the B-foilwith an opposite polarity does not produce any appreciable improvementover discharging the B-foil. It should be noted, however, that in theexample involving charging of the B-foil, the filter material had alower basis weight, viz., 170 g/m². On an equal weight basis, the gainin penetration by opposite charging would be higher (c.f., also thedifferences in pressure losses). This point is demonstrated in the valueof Q.

It is known that a positive corona contains only very few negative ions(10⁻³) times the the number of the positive ones) and vice versa (see R.S. Sigmond in "Electrical Breakdown of Gases", page 361, Wiley, NewYork, 1978 edited by J. M. Meek and J. D. Cragg). Fibers charged freelyon a blocking foil with a positive corona are therefore expected tocarry a unipolar positive charge. Surprisingly, however, the fibers arenot charged unipolarly, but nearly bipolarly, probably owing to afortuitous charging process with a polarity opposite to the chargingpolarity. Said opposite charging probably occurs when the filtermaterial is no longer subjected to the corona charging and is removedfrom the blocking foil.

The bipolarity obtained is favorable because charged particles are thentrapped effectively regardless of the sign of their charge. Moreover,bipolarity also favors the capture of uncharged particles, because itproduces strongly inhomogeneous electrostatic fields.

However, it has been found that fibers charged freely or with mechanicalcompression by application of a positive or negative corona, do carry asurplus of positive or negative charges, respectively. The bipolarityseems to be more balanced when the fibers are charged in a closed spaceunder an overpressure or underpressure.

The bipolarity of the fibers can be improved in free charging bysubjecting the dielectric material, first to a corona with a polarityopposite to the corona polarity applied during the final charging. Saidfree charging is hereafter referred to as pre-filling. Table J showsthat pre-filling improves the penetration results.

                  TABLE J                                                         ______________________________________                                                                       penetration                                                basis   pressure   NaCl at 20                                                 weight  drop       cm/sec  Q                                      pre-filling g/m.sup.2                                                                             Pa         %       Pa.sup.-1                              ______________________________________                                        none        142     18         30      .067                                   with ions of                                                                              140     19         24      .075                                   opposite polarity                                                             ______________________________________                                    

Although an AC corona produces both positive and negative ions itappears, surprisingly, that prefilling by means of an AC corona insteadof a DC corona gives about the same result.

The results of Table J were obtained upon charging polypropylene splitfibers in four layers at 25° C. for 1 second at corona voltages of about±7 kV and with a 2 micrometer thick Mylar blocking foil. Pre-filling wasachieved by charging on an apparatus of the type shown in FIG. 1 butwithout a blocking foil. The actual charging was then carried out with ablocking foil.

So far, the invention has been described as a discontinuous process, butthe methods described are preferably carried out continuously.

FIG. 2 shows an embodiment of an apparatus for carrying out the methodaccording to the invention continuously. In said apparatus, theseparating foil is formed by an endless belt 11 of a substantiallyclosed dielectric foil running over rollers 12-15. These rollers arefree to rotate in a frame (not shown). The upper part 16 of the endlessbelt 11 runs through a double corona device which has, on each side ofthe part 16, the positive corona plasma 17 and the negative coronaplasma 18, respectively. The coronas 17 and 18 are produced by thetungsten wires 19 and 20 connected to high voltage source (not shown) of+7 kV and -7 kV, respectively. At the side facing away from the coronasthe grounded plates 21 and 22 are located. The dielectric material withits open structure, in particular the fiber web 23, is transportedbetween the two coronas in contact with part 16 of the endless belt 11.The tungsten wires 19 and 20 are perpendicular to the feeding directionfor the purpose of achieving a uniform charging of the fiber web 23.

The apparatus shown in FIG. 3, for the continuous charging of fiber web23, is similar to the apparatus of FIG. 2 with respect to the chargingoperation. Consequently, corresponding parts bear the same referencenumber. In the apparatus of FIG. 3 an additional corona device is usedin which part 25 is discharged with an AC corona produced by thetungsten wires 26 and 27 or charged with a polarity opposite to thatproduced by the upper corona device. In the additional lower coronadevice, the grounded plates 28 and 29 can be seen again.

The endless belt 11 of the apparatuses shown in FIGS. 2 and 3 may beformed by a grounded metal belt whose outer surface is covered withdielectric material. In this arrangement, the corona wires 20 and 26with the grounded plates 22 and 28 are omitted. Preferably, the coveradheres to the surface of the belt. If desired, the endless belt 11 mayconsist of dielectric foil metallized on its inner side.

In the apparatus shown in FIG. 4, a fiber web 30 is charged in acontinuous process by means of a one-sided corona device. Said fiber web30 is guided over a rotating metal roller 31 which is grounded. Theseparating foil is put on the circumference of the roller 31 andpreferably adheres as a cover to its surface. Above that part of thefiber web 30 that lies against the foil or cover, a corona device withcorona wires 32 is located. Connected to a voltage source (not shown) ofe.g., +7 kV, the corona wires 32 produce a positive corona plasma 33 forcharging the fiber web 30. On the side of the corona wires 32 facingaway from the corona, a grounded metal plate 34 is placed. It is clearthat the fiber web can also be charged with a negative corona, for whichpurpose a negative voltage source of, e.g., -7 kV has to be connected tothe corona wires 32.

Although not shown, the blocking foil on the roller is discharged orcounterpoled continuously, e.g., with a corona device.

The separating foil may also be placed between the fiber web 30 and thecorona plasma 33, said foil in this case being guided, as an endlessbelt made from a substantially closed dielectric material, over a numberof rollers in a way not shown. Preferably, the fiber web 30 is pressedwith the foil belt against the metal roller, e.g., by means of rollers(not shown). The resulting compression of the fiber web markedlyimproves its charging.

The same position of the foil and the pressure it exerts in thedirection of the fiber web may also be applied in the apparatusesaccording to FIGS. 2 and 3, when the endless belt 11 consists of agrounded metal belt.

Further, the compression of the fiber web may be achieved by stretchinga belt of gauze material over the web in a way not shown. For thatpurpose, an endless belt of gauze material is guided over rotatingrollers. That part of said belt engaging the web is pressed against itby pressing the rollers on both sides of said part in the direction ofthe web.

FIG. 5 is a diagram of an embodiment of an apparatus suitable forcontinuous charging of packages of filter material. The filter material112 wrapped on a reel 113 is unwound from that reel and supplied to avacuum packing device 114 through a supply device (not shown). A blownfoil tube 115 wound on reel 116, is unwound therefrom and supplied tothe packing device 114 through a supply device, likewise not shown. Insaid packing device 114, the supplied filter material 112 and blown foil115 are vacuum-packed. Band 117, thus fabricated of packages of filtermaterial, is fed through a corona device comprising the corona wires 118and 119 and the grounded metal plates 120 and 121. The voltagesindicated by + and - on the corona wires 118 and 119, respectively aregenerated by voltage sources (not shown) of e.g., +7 kV and -7 kV,respectively. The band of packages 117 may be wound on a reel forstorage or transport to the consumer. During storage or transport thefilter material is protected from moisture and dust by its packing.Packages can of course, be cut from band 117 after leaving the coronadevice, and stored or transported separately. After removal of the blownfoil from the packages, the filter material is immediately available foruse in filters.

In the apparatus shown in FIG. 5, the filter material is enclosedbetween two foils and charged in a two-sided corona device. However, thefilter material can also be charged between a blocking foil and agrounded metal counter-electrode with either a positive or a negativecorona. The results are given in Table K.

                  TABLE K                                                         ______________________________________                                                                             corrected                                                     pres-   penetration                                                                           penetration                                           basis   sure    NaCl at 20                                                                            NaCl at 20                               charging method                                                                            weight  drop    cm/sec. cm/sec                                   for filter mat                                                                             g/m.sup.2                                                                             Pa      %       %                                        ______________________________________                                        between 2 blocking                                                                         178     22       6      6                                        foils and with                                                                2 coronas                                                                     between 1 blocking                                                                         135     20      12      6.1                                      foil and metal                                                                counterelectrode and                                                          with 1 corona                                                                 ______________________________________                                    

Table K lists the penetration results of split fiber filters charged ina vacuum (in four layers) in two ways. The charging was carried out withcorona voltages of ±7 kV, at 25° C. in 1 second and with a 2 micrometerthick Mylar foil. Table K compares at the same time two corona chargingmethods, viz., charging between two foils, with two coronas, andcharging between a blocking foil and a grounded metal counter-electrode,with one corona. A quick glance shows that the second charging is lessefficient, because the measured salt penetration is higher. However, theweb weight in this case is 1.3 times lower. If a correction is madeusing equation (2), the salt penetrations are quite similar.

In the apparatus shown in FIG. 6, fiber web 122 is charged in acontinuous process by means of a one-sided corona device and with oneblocking foil. Said fiber web 122 is guided over a rotatable andgrounded metal drum 123. A corona device with corona wires 124 ismounted above that portion of the fiber web 122 which lies against drum123. The corona wires 124, connected to a voltage source of, e.g., +7 kV(not shown), produce a positive corona plasma 125 for charging fiber web122. A grounded metal plate 126 is placed at that side of the coronawires 124 that lies opposite the corona plasma. It is clear that fiberweb 122 can also be charged with a negative corona, for which purpose anegative voltage source of, e.g., -7 kV, is connected to the coronawires 124.

Separating foil 127 is placed between fiber web 122 and corona plasma125, said foil being guided over rollers 128 and 129 as an endless beltof substantially closed dielectric foil. Preferably, that part of thefoil belt which adjoins fiber web 122 presses against this web, e.g., byforcing rollers 128 and 129 down. The fiber web is thereby compressedwhich improves the charging. This can be improved further by dischargingthe separating foil 127, or by continuously poling it with an oppositepolarity by means of the device 130. Said device may contain either anAC corona device, or a corona device imparting to the belt a polarityopposite to that of the corona wires 124.

Still better results are achieved when the charging is carried out in apartial vacuum. For this purpose, drum 123 is provided with holes 131.The interior of drum 123 contains a stationary body 132 provided withlabyrinth sealings 133 and 134 near the inner surface of drum 123.Suction space 135 is defined by the inner wall of drum 123 and a recess136 in the surface of body 132.

Suction space 135 and the labyrinth sealings 133 and 134 extend in thedirection of the axis of drum 123. The pressure in suction space 135 isreduced by a vacuum pump via a suction conduit (both not shown). Throughholes 131 in drum 123 suction space 135 communicates with space 137 forfeeding through fiber web 122. The underpressure established in thefeed-through space is nearly equal to that in suction space 135. Thereduced pressure in feed through space 137 forces foil 127 against drum123, compressing the fiber web and causing the charging to take place ina partial vacuum.

Further, rollers 138 and 139 are provided for guiding fiber web 122 inthe desired direction.

Although the above is largely concerned with split fibers, the methodsof the invention also allow charging of other fiber structures, e.g.,melt blown fibers and others.

When electret split fibers are used in dust filters, it is desirablethat the fibers are crimped in order to increase their dust-catchingcapacity. The method described in Dutch patent application 7614376 inwhich a closed foil, e.g., blown foil is charged on both sides provideshighly bipolarly charged fibers. However, the attraction of oppositelycharged surfaces of the fibers causes an undesirable attraction forcethat opposes the crimping of, e.g., bicomponent split fibers. The fibersmanufactured by the invention are slightly charged unipolarly andthereby mutually repel one another so that crimping is not prevented.Therefore, the fibers are preferably charged after crimping.

Fiber webs obtained by fibrillation of foils, in particular blown foils,can be charged in a number of ways. For instance, from blown foil tubetwo flat foils can be obtained. After fibrillation, one foil can bebombarded with positive charges and the other with negative charges,whereupon they are assembled into a so-called macrobipolar filter web.Alternatively, one of the foils can be left unfibrillated in order toserve as the separating foil for the second fibrillated foil.

We claim:
 1. A method for manufacturing an electret filter medium from adielectric material with an open or porous structure, comprising thesteps of:continuously feeding a web of said dielectric material with asubstantially closed non-fibrous dielectric foil adjacent to at leastone major face thereof into a corona discharge device; reducing thethickness of said web of dielectric material to achieve upon charging aweb of superior filtration efficiency; and charging said web of reducedthickness dielectric material by means of a corona discharge.
 2. Amethod according to claim 1 wherein the thickness of the dielectricmaterial is reduced by compression.
 3. A method according to claim 2wherein said compression is achieved by applying a pneumaticoverpressure to said substantially closed dielectric foil overlying saidweb of dielectric material.
 4. A method according to claim 2 whereinsaid compression is achieved by laying an open gauze or net onto saiddielectric material and pressing said open gauze or net against saiddielectric material.
 5. A method according to claim 2 wherein thethickness of the dielectric material is temporarily reduced duringcharging.
 6. A method according to claim 2 wherein the thickness of thedielectric material is permanently reduced prior to charging.
 7. Amethod according to claim 1 wherein the dielectric material istemporarily stretched during charging.
 8. A method according to claim 7wherein the dielectric material is temporarily stretched along itslength.
 9. A method according to claim 7 wherein the dielectric materialis temporarily stretched along its width.
 10. A method according toclaim 7 wherein the dielectric material is temporarily bidirectionallystretched.
 11. A method according to claim 1 wherein the thickness ofthe dielectric material is reduced by placing said dielectric materialwithin a substantially gas-tight enclosure, at least one major boundaryof which is flexible and extends along a major face of said dielectricmaterial, and reducing the pressure within said substantially gas-tightenclosure.
 12. A method according to claim 1 wherein the substantiallyclosed non-fibrous dielectric foil is discharged between successivecharging operations.
 13. A method according to claim 1 wherein thesubstantially closed non-fibrous dielectric foil is charged with apolarity opposite to the polarity used for charging said web ofdielectric material.
 14. A method according to claim 1 wherein the webof dielectric material is subjected to a precharging with a polarityopposite to the polarity used for the final charging of said web ofdielectric material.
 15. A method according to claim 1 wherein the webof dielectric material is subjected to AC-corona before final charging.16. A method according to claim 1 comprising the additional step ofstacking a plurality of charged webs of said dielectric material to forma composite electret filter medium.
 17. A method according to claim 16wherein said plurality of charged webs is unified by needle tacking orheat sealing.
 18. A method according to claim 1 wherein the thickness ofthe web of dielectric material is permanently reduced by preshaping saidweb.
 19. A method according to claim 1 wherein the dielectric materialcomprises a web of dielectric fibers.
 20. A method according to claim 19wherein the web of dielectric fibers comprises a mixture of coarsefibers and fine fibers.
 21. A method according to claim 19 wherein theweb of dielectric material comprises a stack of a plurality ofindividual webs formed of coarse dielectric fibers and fine dielectricfibers.